There have been numerous applications for double and triple resonance circuits in NMR of solid samples with high-power decoupling for the past three decades. The most common application is irradiating at the proton (.sup.1 H) high frequency (HF) resonance to decouple its dipolar broadening effects while observing Bloch decays on a nuclide of lower magnetogyric ratio such as .sup.13 C at a low frequency (LF). Other common examples of multiple resonance circuits include cross-polarization (CP) and Rotational Echo Double Resonance (REDOR). A number of specialized techniques--Dynamic Angle Spinning (DAS), Double Rotation (DOR), single-crystal, etc.) have been developed during the past decade to improve spectral resolution of solid samples, and the probe requirements of some of these are reviewed briefly by Doty in `Solid State NMR Probe Design`, The NMR Encyclopedia, Vol. 6, Wiley Press, 1996. More recently, inverse NMR techniques on liquids are being applied to nuclides with large chemical shifts, where high power decoupling is sometimes needed on the LF channel.
For experiments on solid samples at high static magnetic field B.sub.0 (greater than 7 T), where large rf fields B.sub.1 are required, typical voltages across the sample coil are 2 to 6 kV. In U.S. Pat. No. 5,162,739, Doty discloses an improved method of balancing a double-tuned high-power circuit so that proton B.sub.1 can exceed 2 mT (.gamma.B.sub.1 =85 kHz) at 400 MHz with 0.3 ml samples in a Cross Polarization Magic Angle Spinning (CPMAS) probe that is broadbanded on the LF channel and uses a sample spinner such as the one described by Doty et al in U.S. Pat. No. 5,508,615 (note the extensive list of typographical corrections). Broadband (multinuclear) triple-resonance circuits as disclosed by Doty in U.S. Pat. No. 5,424,645 have provided multinuclear tuning capability on an LF and Mid-Frequency (MF) channel with high-power .sup.1 H decoupling for various NMR experiments such as REDOR and double CP.
The above referenced prior art typically achieves MF and LF rf efficiencies .eta..sub.E of 35% to 60% (percentage of power delivered to sample coil) for 5 mm solenoidal sample coils (sample volume about 0.1 ml) at 12 to 7 T, but HF efficiencies of balanced coils are typically under 35% at 300 MHz and decrease to under 20% at 600 MHz. It has recently been shown that irradiating with very high decoupling fields (up to 150 kHz) for longer acquisition times (up to 300 ms) can achieve nearly liquid-like resolution in many complex solid samples if other factors (susceptibility effects, sample preparation, magic angle precision, thermal gradients, and spinning speed) are also properly addressed. However, prior art coils and circuits are not capable of this level of proton decoupling above 200 MHz unless major sacrifices (factors of 2 to 4) are made in MF and LF sensitivity. The instant invention permits, without a corresponding loss in sensitivity, approximately a factor of three increase in both decoupling efficiency and maximum decoupling B.sub.1 above 700 MHz, and some advantages are realized as low as 200 MHz.
For the past five decades, NMR probes for solid samples have almost exclusively utilized solenoidal sample coils, as the solenoid achieves higher filling factor .eta..sub.F and Q for small samples than the saddle coil and the related transverse resonators usually used in high-resolution NMR probes for liquids. Moreover, the multi-tuned single-solenoid may simplify CP with uncompensated coils. However, transverse coils in combination with solenoids permit geometric isolation by virtue of their zero mutual inductance, and this allows the use of circuits with considerably higher rf efficiency. Consequently, several attempts have been made over the past decade to combine transverse coils and solenoids advantageously for multiple-resonance solids NMR at high field, as had been done from very early days for liquids NMR in low-field magnets, as disclosed by Anderson in U.S. Pat. No. 3,771,055.
Known prior art attempts at combining transverse resonators with solenoids in solids NMR probes have all placed the transverse coil outside the solenoid, primarily because the transverse coil has been used for .sup.1 H decoupling and the .eta..sub.F Q product is less important at higher frequencies. Thus, the inner solenoid is easily optimized for maximum LF and/or MF sensitivity. However, such prior art attempts have not been satisfactory because solids NMR at high fields is generally applied to sample sizes below 6 mm, where it is critical that spacing between coils be minimized to maintain acceptable .eta..sub.F on the outer coil. While flat wire may be preferred for single-coil solenoids of more than five turns, the three-turn to five-turn solenoids generally needed at very high fields have better Q and B.sub.1 homogeneity if at least the final turn at each end is of heavy, round wire. Thus, considerable radial space is desired for the solenoid. Moreover, it is very difficult to affix a coil like this to the inside of a coil former in a way that achieves both high B.sub.1 and B.sub.0 homogeneity.
In one embodiment of the instant invention, a thin transverse .sup.1 H decoupling coil is affixed in intimate thermal contact on the inside of a ceramic coilform of high thermal conductivity, thereby allowing this coil to handle high-energy pulses without significantly reducing the filling factor of an outer solenoidal rf coil. Relatively minor modifications in the Supersonic MAS Spinner of U.S. Pat. No. 5,508,615 are required to accommodate an inner transverse coil and an outer solenoidal coil, although more extensive modifications would be required in some designs currently in use. Generally speaking, rf efficiency of the .sup.1 H decoupling circuit is greatly increased, especially at high fields, while Q is somewhat decreased. The LF and MF frequencies often see little change in sensitivity, as efficiency improvements are offset by a loss in filling factor. However, very high LF and MF field strengths may be achieved more readily, as it is now easier to balance the LF and MF circuits, and this results in a considerable increase in sensitivity in multiple-quantum techniques.
The concepts disclosed herein enabling greatly improved decoupling efficiency in broadband double resonance NMR spinning techniques for solids are also applicable to non-spinning techniques (single crystal and wide-line) and to doubly-broadband triple and quad-resonance circuits for NMR techniques requiring three or four high-power channels. If sufficient attention is paid to magnetic compensation of the coils and advantageous spinner symmetries and if leak-proof susceptibility-matched plugs are used in the rotor, B.sub.0 homogeneity may be improved by an order of magnitude, and the novel coil arrangement may be applied to MAS of liquid samples, as used in combinatorial chemistry techniques for drug design. Finally, some of the novel techniques may be applied to high-power decoupling (especially X-nucleus) in conventional high-field high-resolution NMR of liquid samples.